1. Field of the Invention
The present invention relates to a method of fabricating a lead frame for a semiconductor device fabricated by resin-sealing a semiconductor chip and a method of fabricating a semiconductor device using the same, and such a semiconductor device and a portable apparatus and an electronic apparatus comprising the same.
2. Description of Related Art
In order to high-density mount a semiconductor device on a wiring substrate, a package for high-density mounting allowing surface-mounting on the wiring substrate by eliminating extension of a lead from a mold resin package and exposing a lead (a terminal portion electrically connected to a semiconductor chip) of a lead frame to a lower surface of the package has been conventionally used. Known as such a package for high-density mounting have been leadless packages such as QFN (Quad Flat Non-leaded Package) and SON (Small Outlined Non-leaded Package).
In the packages in these forms, a lower surface of the lead sealed with mold resin, together with the semiconductor chip, is exposed to the lower surface of the package, so that the lead easily slips off the mold resin. Therefore, the lead can be prevented from slipping off by forming the lead into a reverse-tapered shape as disclosed in U.S. Pat. No. 6,664,133, rough-surfacing an upper surface of the lead as disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2003-158234, or providing an offset in a part of the lead as disclosed in U.S. Pat. No. 6,700,189.
In order to form the lead having such a cross-sectional shape, the lead frame has been conventionally processed by etching. However, a long time is required for the processing, and the cost is increased. Therefore, the fabrication of the lead frame using a precision press mold (die) has been recently proposed, as disclosed in U.S. Pat. No. 6,664,133, for example.
In the method disclosed in U.S. Pat. No. 6,664,133, the lead frame is fabricated by subjecting a metal plate serving as a material for the lead frame to punching processing and pressing processing using a punch from the side of a lower surface of the lead frame, and stepped shapes for slipping prevention are respectively formed at a front end and a side surface of the lead.
In the case of the punching processing from the side of the lower surface of the lead frame, however, a sag occurs at a side edge on the lower surface of the lead so that the lead has a semi-round shape in cross section. The sag cannot be eliminated even by forming the stepped shape corresponding to the side surface of the lead by the subsequent pressing processing, so that the lower surface of the lead forms a curved surface projected downward.
When the lead frame, together with the semiconductor chip, is resin-sealed, therefore, the sealing resin may detour toward the lower surface of the lead to cover a part of the lower surface of the lead which should function as an outer lead. This results in inferior contact and defective products.
This problem can be solved by performing first punching processing from the side of an upper surface of the lead frame. In such a method, however, a burr projects toward the lower surface of the lead, which still causes the defective products.